Method of vapor depositing coatings of aluminum



Jan. 17, 1956 A. R. WEINRICH 2,731,366

METHOD OF VAPOR DEPOSITING COATINGS OF ALUMINUM Original Filed Dec. 28, 1948 226%? adam 1c Gttornega and entitled .shownas a whole at 11. the form of a bell-jar or the like having a dome-l'ike or semi-spherical top'portion -or enclosed end'and a'bot- United States Patent 2,731,366 TMETHOD OF VAPOR DEPOSITING. COATINGS OF ALUMINUM ArthurR. Weinrich, Dallas,"Tex., assignor to Libbey- Owens-Ford Glass Company, Toledo, Ohio, a corporation of Ohio Claims. (Cl..117--'7) This invention relates broadly to improvements in metal evaporation and, more particularly to the. thermal evaporation of aluminum alloys in theforming of mirrors and other coatings.

-The presentapplicationis a division of mytcopending application Serial No. 67,609, .filed December 28, 1948,

Aluminum .Base Alloy for Metal Evapora- .tion,which is now Patent No. 2,589,175.

'Inmetal evaporation, the metal to be evaporated is usually hung on a filament of tungsten, molybdenum, tantalum, or columbiunr as .smallloops or pieces of wire. Electric current is thenpassed through thefilament to heat it and the wire pieces are. melted whereupon the fused metal clings to the filament and evaporates as theheating is continued. i

In the application of mirror and .other coatings by evaporationwithin a vacuumas known .inthe thermal evaporation art, considerable spoilage isleucountered by small chunks or particles of metal which tend to spatter or fly oil the electric filament and cause rough spots upon the mirror. or other article beingcoated, rendering it unfit for commercialuse. This highly objectionable condition is frequently encountered when .usingpure or commercially pure aluminumand is customarilyreferred to in the art as spitting.

The primary object .oftthis inventionis toimprove the thermal evaporation of aluminum and its alloys so as to minimize, if not entirely eliminate, objectionable spattering or spitting ofthemetal during evaporation.

Anotherobject of the invention is to overcome spitting of the metal during evaporation by the provision of a special alloy of aluminumcontaining a relatively. small amount of tungsten or a combination of tungsten and molybdenum and the balance substantially aluminum.

The use of such a special alloy of aluminum with tungsten or with tungsten and molybdenumhas been found to eliminate the spoilage from spitting or explosive boiling such as is found to a considerable degree when evaporating pure or commercially pure aluminum.

Other objects and advantages of the invention will be apparent from the following description and claims.

In the drawings: Fig. 1 is a perspective view, partially broken. away,

.of one type of thermal evaporation apparatus;

Fig.2. is a perspective view of a portion of an elec tric filament (showing the application of the. metal alloy toseveral'of the coils thereofpand Figs, 3, 4, 5 and 6 are detail diagrammaticviews illustrating the several phases of wetting or coating of the filament by the alloy of this invention.

vSuitab'le apparatus which may be employed in carrying out the invention is illustrated in Fig. l and comprises a supporting base 10 upon'which is mounted a housing The housing 11 may be in tom open end having a surrounding flange or projection 12 which is adapted to rest upon the upper surface of the supporting base 10.

Within the chamber provided by the housing 11 is a suitable work support 13 for supporting a Work piece such as a plate or piece of glass, plastic, plaster, paper, porcelain, metal, or the 1ike'14, in upright position.

Also located within the chamber and mounted upon the supporting base10 is a pair of upright supporting posts 15 between which is carried in substantially horizontal position an electric filament 16. The filament, as shown, is in the form of a coiledwire made of tungsten, molybdenum, columbium, or tantalum, whose opposite ends are attached to brackets 17 mounted upon the supporting posts 15 and adjustablethereon so as to vary the position or location of the filament 16 with relation to the supporting base 10.

The chamber provided by the housing 11 may, if desired, be completely evacuated of air through outlet pipe or conduit 18 and have ahigh vacuum created therein by means of suitable air evacuating and vacuum creating means such as a pump (not shown).

in accordance with the invention, which is carried out within the chambered housing 11, short pieces or loops 19 of the special aluminum alloy herein provided and consisting of a relatively small amount of tungsten, or a combination of tungsten and molybdenum with the balance substantially all aluminum, are bent and hung on the convolutions 16a of'the filament16 in the manner shown.

When pure or commercially pure aluminum such as 98.5% aluminum is appliedto a thermal evaporation filament and the filament energized by applying a heavy current thereto, the small pieces of aluminum receive their heat by contact with the'filament and by heat conduction from the filament down through the metal pieces. Under such circumstances, the filament is invariably at a much higher temperature than the aluminum at the time the aluminum reaches its melting point of 658 centigrade. Thus, depending upon the construction of the filament and the amount of electrical current applied thereto, the filament. may be at a temperature in the range of 1000 to 2000 degrees centigrade, in order to transfer sufi'icient heat to the aluminum and to evaporate it in a reasonable time.

Pure and commercially pure molten aluminum wets by capillary attraction the metals tungsten, molybdenum, tantalum and columbiurn which are normally employed as evaporation filaments and such wetting occurs very rapidly. In fact, when the aluminumypieces applied to the filaments become molten, they wetthe filament so rapidly that the relatively cold molten aluminum flows out instantly over the much hotter filament surfaces originally not contacted by the aluminum pieces and as these hotter surfaces are above. the temperatures required for forming aluminum vapor in the vacuums employed, the

advancing molten aluminum is explosively vaporized.

That portion of'the advancing molten aluminum flowing by capillary attraction over and in direct contact with the very much hotter filament surfaces is the portion which is immediately vaporized and which acts to blow away the overlyingliquid aluminum by the explosive "boiling conditions thus generated. This spitting results upon melting, supplies molten metal to the hot filament surfaces not originally contacted thereby at a relatively much slower rate. tungsten or tungsten and molybdenum acts to retard the speed at which the aluminum alloy feeds out to the filament. While the alloy seems to be of the same surface tension and good wetting character for the filament metals as with pure or commercially pure aluminum, the alloy does not melt as awhole to a simple molten liquid as with pure or commercially pure aluminum, but gives a melt in which many small crystals of an unmelted material are present. a

More particularly, the alloy melts first to a semi-pasty mass as indicated at 19a in Fig. 3, and such pasty character retards the flow of molten metal by capillary attraction onto the hotter surrounding filament surfaces. This retardation also permits a longer time for heat to flow from the filament by conduction into the pasty mass 19a and its temperature is thus raised to a higher temperature so that the mass temperature and any small amounts of molten metal leaking out ofthe mass onto the hotter filament surfaces are relatively closer to the'actual temperature of such surrounding filament surfaces. By re ducing the temperature differential between the molten metal mass 19a and the surrounding portions of the filament upon which it later moves out upon, the danger of explosive boiling or spitting is minimized.

In Fig. 4, the pasty melted alloy mass 19b has become hotter than as shown in Fig. 3 and from such a pasty mass the molten phase begins to leak out upon the surrounding filament which it wets. The viscosity conditions in the pasty mass are sufficiently high, however, that the rate of leakage of the molten phase from the latter is controlled and relatively slow so that the amount of molten liquid moving out from the mass onto the surrounding filament surfaces results in only a very thin film of such liquid being present upon the filament surface, and the boiling action in such film as the liquid metal evaporates results only in completely vaporized metal and no unvaporized liquid metal is blown ofi? or spitted.

As the temperature of the metal alloy mass is being continuously raised by heat flow from the surrounding filament, such heat movement from the filament into the mass is cooling the adjacent portions of the filament somewhat and the filament and mass are progressively rapidly approaching each other in temperature, or the temperature differential between the two is becoming less and less. As the mass becomes hotter, more and more of thealloy appears to become fluid and the wetting of the filament proceeds through stages such as indicated at 19c in Fig. 5, and 19d in Fig. 6. In this manner, the improved alloys herein provided slow down the rate of application of the molten metal to the hot filaments and spitting is avoided.

Aluminum forms with tungsten or with molybdenum intermetallic'compounds which are of very high melting point, and in alloys of these materials the intermetallic compounds are present as fine crystals generally of a needle like nature. On heating such aluminum alloys containing small amounts of these metals, the excess aluminum over that needed to form such intermetallic compounds begins to become fluid as temperatures beyondthe melting point of aluminum are reached. The intermetal- .lic compound materials, however, do not melt until much higher temperatures are reached and the presence of these crystalline materials in the molten aluminum'phase results in the pasty or highly viscous nature of the molten alloys. As the temperatures are raised beyond the melting point of pure or commercially pure aluminum and the mass approaches the temperature of the filament surfaces, the viscosity of the molten aluminum alloy decreases and finally the intermetallic compounds melt, and as the crystals disappear the pasty mass becomes more liquid and fluid.

As the fluid aluminum or mixture of melted alumi- In other words, the inclusion of the num and intermetallic compounds or molten alloy flows onto the surrounding filament surfaces, the aluminum is vaporized into the evacuated chamber and deposits upon the article 14 to give a coating of aluminum. The aluminum present in the intermetallic compounds is also apparently distilled away from such compound and the tungsten or molybdenum set free remains upon the filament. Thus, while an alloy of aluminum is provided for evaporation purposes the deposits formed are of pure aluminum and show all of the high reflectivity of pure aluminum. By way of contrast, the aluminum alloys, due to the intermetallic compounds, are grey and dull and will not take a polish such as is true of pure aluminum. By the use of the special aluminum alloys herein provided, aluminum mirrors of equal high reflectivity to those secured when evaporating pure aluminum are obtained. i

In the thermal evaporation-art, it has been suggested that the cause of spitting has been small amounts of air or other gases trapped in thealuminum, during the casting of the original billets, becoming heated andfreed explosively at the time of the melting of the aluminum pieces in the vacuum. This, however, does not seem'to be the case, but rather is caused by the above described temperature differential efiects of the aluminum pieces and electric filament.

In general, it has been found that suitable aluminum alloys accomplishing the objects of this invention in producing coatings of aluminum by thermal evaporation are those containing tungsten in an amount of less than 10% and the balance substantially aluminum. Further, it has been discovered that the amount of tungsten in the aluminum alloy may be in the amount of 0.25% ,to 6.0%, with good results being obtained with an alloy of aluminum containing 1.0% tungsten. Forming of the alloys either into a wire or a pellet is more readily carried out with the smaller amounts of the alloying tungsten or combination of tungsten and molybdenum as the presence of the intermetallic compounds in the aluminum alloys, cause these alloys to be much harder than pure aluminum.

When a combination of tungsten and molybdenumare employed in an alloy with aluminum, it has been found desirable that the combined amount of the twov alloying metals be less than 10% with the balance of the alloy substantially aluminum. For example, the tungsten in an alloy of this type may be of at least 0.1% and the total of the two metals tungsten and molybdenum varied within a combined amount in the alloy of up to6.0% with the balance substantially aluminum. These alloys are very useful and a more specific alloy of this type would be one containing 2% tungsten and 4% molybdenum, the balance being substantially aluminum.

Other specific aluminum alloy compositions which may be used in following the invention are by way of example:

l. Tungsten 0.25%, balance aluminum 2. Tungsten 5.5%, balance aluminum 3. Tungsten 0.1%, molybdenum 0.8%, balance aluminum 4. Tungsten 2%, molybdenum 3.7%, balance aluminum In stating that the balance of the alloy, besides the tungsten, or combined tungsten and molybdenum content, is substantially aluminum, it is the intent herein to signify that such balance is pure aluminum or commercially pure aluminum,'the alloy being such as would result from the addition of the tungsten or combined tungsten and molybdenum to such forms of aluminum. The other elements 7 of aluminum on a surface, which consists inproviding a filament selected from the group consisting of tungsten, molybdenum, tantalum and columbium, placing on said filament an aluminum alloy containing between 0.25% and 10% tungsten and the balance substantially aluminum, said tungsten coacting with said aluminum and said filament to retard the speed at which the aluminum spreads out upon the filament when heated preliminarily to being evaporated, and heating said aluminum alloy to evaporate the aluminum component thereof from said filament onto a surface to be coated.

2. A method according to claim 1, wherein the aluminum is alloyed with about .25% to 6.0% tungsten.

3. A method according to claim 1, wherein the alumimum is alloyed with about 1.0% of tungsten.

4. The method of providing a smooth uniform coating of aluminum on a surface, comprising providing a filament selected from the group consisting of tungsten, molybdenum, tantalum and columbium, placing on said filament an aluminum alloy containing a combined amount of tungsten and molybdenum, with the tungsten being at least 0.1% of the alloy and the molybdenum being at least 0.8% of the alloy, the combined tungsten and molybdenum being less than 10% and the balance substantially aluminum, said tungsten and molybdenum coacting with said aluminum and said filament to retard the speed at which the aluminum spreads out upon the filament preliminarily to being evaporated from said filament, and heating said aluminum alloy to evaporate the aluminum component thereof from said filament onto a surface to be coated.

5. A method according to claim 4, wherein the aluminum is alloyed with about 2.0% tungsten and about 4.0% molybdenum.

References Cited in the file of this patent UNITED STATES PATENTS 2,363,781 Ferguson Nov. 28, 1944 2,382,432 McManus et a1 Aug. 14, 1945 2,589,175 Weinrich Mar. 11, 1952 OTHER REFERENCES Hansen: Aufbau der Zweistofilegierungen, Photo- Lithograph-Production, Edwards Bros., Ann Arbor, Mich. 1943, page 135.

Mondolfo: Metallography of Aluminum Alloys, published by John Wiley & Sons, Inc., 1943, page 30. 

1. THE METHOD OF PROVIDING A SMOOTH UNIFORM COATING OF ALUMINUM ON A SURFACE, WHICH CONSISTS IN PROVIDING A FILAMENT SELECTED FROM THE GROUP CONSISTING OF TUNGSTEN, MOLYBEDENUM, TANTALUM AND COLUMBIUM, PLACING ON SAID FILAMENT AN ALUMINUM ALLOY CONTAINING BETWEEN 0.25% AND 10% TUNGSTEN AND THE BALANCE SUBSTANTIALLY ALUMINUM, SAID TUNGSTEN COACTING WITH SAID ALUMINUM AND SAID FILAMENT O RETARD THE SPEED AT WHICH THE ALUMINUM SPREADS OUT UPON THE FILAMENT WHEN HEATED PRELIMINARILY TO BEING EVAPORATED, AND HEATING SAID ALUMINUM ALLOY TO EVAPORATE THE ALUMINUM COMPONENT THEREOF FROM SAID FILAMENT ONT A SURFACE TO BE COATED. 